Referring to FIG. 1A, the power amplifier (PA) 10 is a circuit and/or device with inherently nonlinear input-output characteristics. As shown in FIG. 1B, the PA output voltage Vout as a function of PA input voltage Vin is generally linear to a point, and as Vin increases beyond that point, the curve becomes logarithmic (i.e., the increase in Vout per unit increase in Vin decreases; that is, d(Vout)/d(Vin) is negative). Thus, while the degree of nonlinearity may vary depending on the class of the amplifier, almost all PAs exhibit nonlinearity when operated at mid-to-high range of input power levels, due to saturation effects within the PA. The nonlinearity of the PA is a function of the input power (or input amplitude) and is characterized by the AM/AM and AM/PM curves, which model the distortion of the output amplitude and output phase, respectively, as a function of the input amplitude. Referring now to FIG. 2, when a PA input signal contains two sinusoid waveforms ω0 and ω1, the effects of the nonlinearity appear as intermodulation products (ω0−2ω1) and (ω0+2ω1). As shown in FIG. 3, such intermodulation products result in out-of-band spectral regrowth 20a and 20b and co-channel and/or adjacent channel interference 25, thereby significantly impacting system performance.
Highly linear PAs tend to be power inefficient. Therefore, they are generally not attractive for low power applications, such as wireless devices. As a result, the PAs designed for such applications tend to be power efficient, but they also tend to suffer from poor linearity. One way of avoiding the effect of PA-induced distortions is to operate with the input signal attenuated enough so that the region of operation is restricted to where the PA response is linear. However, this approach generally results in lower output power, and therefore, a lower output power range. The drawback of operating with an attenuated input signal becomes especially severe when the signal has a high peak-to-average power ratio (PAPR). In such a case, constraining the peak power to be within the linear regime of the PA response can adversely impact the average output power.
One preferred approach is to predistort the input signal to the PA to compensate for the non-linear effects. In other words, and as shown in FIG. 4, one designs a predistortion function 30 that is the mathematical inverse of the nonlinear characteristics 40 of the PA, such that the cascade of the predistortion function 30 and the PA results in a roughly linear amplification function of the amplitude of input signal IN, as observed at output OUT. The predistortion function 30 generally comprises prescaling the amplitude and changing the phase of a complex valued signal (e.g., IN) to compensate for the nonlinear amplitude dependent scaling (AM/AM) and phase rotation (AM/PM) effects introduced by the PA.
The predistortion function for a given PA characteristic also corresponds to a given operating point (e.g., desired output power). If the PA characteristic changes, then the predistortion function generally should change. Similarly, if the operating point is changed, but the PA characteristic is not changed, then the predistortion function should be adjusted accordingly. Otherwise, the signal can be overly predistorted for certain (generally higher) amplitudes and could actually result in an effective gain expansion type of nonlinearity.
For example, most conventional analog amplifier systems include a variable gain amplifier (VGA) configured to attenuate or restrict the voltage gain swing of a PA input signal. A VGA often limits the maximum value of Vin, usually to keep the PA in or near the linear region of operation. However, when the input signal Vin has a voltage that exceeds Vinmax (see, e.g., Vin value 60 and its position relative to Vinmax line 50 in FIG. 1B), the VGA reduces the value of Vin at the PA input to Vinmax, which results in the predistortion function (operating on the actual value, rather than the attenuated value, of Vin) overdistorting Vin and introducing gain expansion nonlinearity into the PA.
Thus, a need is felt for a circuit and method for (pre)distortion in an amplifier that provides a generally linear output power function over a range of amplifier operating parameters and a wider range of input power.